1. Field of the Invention
The present invention relates to an actuator for an electrically actuated control valve which is suitable for use in controlling pressure and flow rate of high-pressure fluids such as high-pressure steam in a steam power plant.
2. Description of the Prior Art
A known electrically actuated control valve comprises a casing containing a valve chamber; an inlet and an outlet; a valve seat which is located in the chamber; a movable valve rod supported in the casing and connected to a valve plug; and an actuator which utilizes a D.C. servo-motor to move the valve rod so as to drive the valve plug into and out of contact with the valve seat, thereby controlling the degree of opening of the control valve.
The D.C. servomotor in turn is under the control of a phase-control type controller, which determines the deviation of the actual opening degree from a demand opening degree and drives the D.C. servomotor in the direction required to reduce the deviation. The controller then stops the D.C. servomotor when the deviation has been reduced to zero.
When stopped at the demand opening degree, however, the D.C. servomotor cannot produce torque to balance the load acting on the valve rod. A worm gear train which provides an automatic locking function is therefore incorporated in the system for transmitting torque from the D.C. servomotor to the valve rod. The provision of the worm gear train, however, complicates the construction of the torque-transmitting system and requires the D.C. servomotor to have a large capacity because of the low torque-transmitting efficiency of the worm gear train.
A valve actuator utilizing a D.C. servomotor must also incorporate a torque limit switch in the torque-transmitting system in order to protect the valve members and the torque transmitting system itself from any excessive force which may be caused by any foreign material jammed between the valve plug and the valve seat when the valve is in the fully closed position or in an intermediate open state.
Furthermore, the operating speed is limited and additional maintenance work is necessitated because of the brushes required by the DC motor to rectify the electric current.